feat: add tool choice configuration and update steam handling in Gemini

[mxdlzg]

close #1993

我已确认该 PR 已自测通过，相关截图如下：
（此处放上测试通过的截图，如果不涉及前端改动或从 UI 上无法看出，请放终端启动成功的截图）

1、增加Gemini对tool_choice的支持。
顺便修复：thinking模型回答吞字问题；Gemini已经支持system指令，因此可以移除AddDummyModelMessage。
API资料：https://ai.google.dev/gemini-api/docs/system-instructions?hl=zh-cn&lang=rest

system指令：

tool_choice参数：

thinking模型：

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 27 lines in your changes missing coverage. Please review.

Project coverage is 1.18%. Comparing base (a858292) to head (ad56898).

Files with missing lines	Patch %	Lines
relay/adaptor/gemini/main.go	0.00%	27 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #1996      +/-   ##
========================================
- Coverage   1.18%   1.18%   -0.01%     
========================================
  Files        150     150              
  Lines      12742   12746       +4     
========================================
  Hits         151     151              
- Misses     12577   12581       +4     
  Partials      14      14              

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